1. Field of the Invention
The present invention relates to an improvement of lead-free piezoelectric ceramics.
2. Discussion of Related Art
Piezoelectric ceramics are used in various fields of technology, as a material for filters, actuators, transducers and any other devices capable of converting an electric energy (electric signal) directly into a mechanical energy (mechanical signal). At present, a perovskite compound containing lead is the most commonly used piezoelectric ceramic material. A typical example of such a perovskite compound is lead zirconate titanate (Pb(Zr, Ti)O3). However, the lead zirconate titanate has a problem of environmental pollution caused by manufacture and disposal of a device using this piezoelectric ceramic material, since lead (PbO, in particular) contained in the lead zirconate titanate is harmful. In view of this problem, it has been proposed to use potassium sodium niobate ((K, Na)NbO3) and other lead-free piezoelectric ceramics represented by ANbO3, where “A” represents an alkali metal. An example of the lead-free piezoelectric ceramics is disclosed in JP-A-2000-313664.
Generally, lead-free piezoelectric materials are inferior to lead-containing piezoelectric materials in the piezoelectric properties such as electromechanical coupling factor and mechanical quality factor. Of the lead-free piezoelectric materials, (K, Na)NbO3, for example, has comparatively excellent piezoelectric properties, but is relatively hard to be sintered, so that formation of a highly dense structure of (K, Na)NbO3 having excellent piezoelectric properties requires a hot pressing process, which is disadvantageous in the cost and efficiency of manufacture of the desired device, and in the freedom of design of the configuration and size of the structure. The low degree of sinterability of (K, Na)NbO3 is due to a high degree of volatility of alkali metal components (K, Na) and a low melting point of KNbO3, which tend to cause abnormal growth of grains and make it difficult to raise the firing temperature to a level high enough to assure high densification of the structure.
The lead-free piezoelectric material ANbO3 disclosed in the above-indicated publication JP-A-2000-313664 contains one of Cu, Li and Ta as an additive to increase the sinterability and thereby improve the piezoelectric properties. Where a compound including one of those elements is added to or contained as a solid solution in the lead-free piezoelectric material in a trace amount, the element acts as a sintering agent to improve the sinterability. However, an experimentation described in the above-identified publication demonstrated that the addition of Cu to the piezoelectric material disclosed in the publication to improve the sinterability undesirably resulted in a reduction of the dielectric constant ε, and the addition of Li or Ta undesirably resulted in reductions in the electromechanical coupling factor kp and Curie point. Further, an experimentation conducted by the present inventors confirmed that the addition of Cu (e.g., in the form of CuO) resulted in substantially no improvement in the sinterability.